This invention relates to a video special effects generator.
Various types of special effects generators exist in the prior art, in which a composite video signal is produced consisting of selected parts of two input video signals from two cameras by selectively switching the video signals. The conventional special effects generator is provided with an analogue signal processing circuit including horizontal and vertical sawtooth or parabolic generators which generate analogue potentials proportional to the horizontal and vertical positioning of a scanning spot. Analogue comparators are then arranged to compare various combinations of these analogue signals, and to generate switching signals which operate electronic switches. The electronic switches switch portions of the input video signals into a single output video channel under the control of the relative magnitudes of the analogue signals which are being compared. The horizontal and vertical sawtooth or parabolic generators are usually made up of integrating circuits comprising capacitance and resistance. As a result, the generators are prone to drift with temperature changes and with the aging of components.
In order to overcome the above-described defects, a digital special effects generator has been proposed such as shown in U.S. Pat. No. 3,821,468 and U.S. Pat. No. 3,941,925, in which the generator includes at least a lever counter, a horizontal counter and a digital comparator. The lever counter produces a digital signal corresponding to a position of a manually operated potentiometer, while a horizontal counter counts the predetermined number of pulses, for example, 512 pulses per horizontal line to provide the digital indication of the horizontal position of the scanning line. The contents of the lever counter and the horizontal counter are compared in the comparator which provides a transition signal when the count of the horizontal counter exceeds the value stored in the lever counter. This causes the output video signal to be switched in accordance with the position of the lever counter. A similar lever, counter and comparator may be provided for wipe (or key) in the vertical direction.
A special effects generator according to these patents requires that the horizontal and vertical digital comparators produce the switching signals for the horizontal and vertical wipe (or key) operation, respectively. It is apparent that the construction of the generator becomes more complex because of the addition of the comparators and it is extremely difficult to exactly control the size and speed of the switching signal from the generator.